C-RAN (RAN: Radio Access Network) has been considered as a solution for future cellular wireless communication which can address many challenges the operators are facing, such as large number of base stations, high power consumption, rapidly increased CAPEX/OPEX, low base station utilization rate, etc. C-RAN is mainly characterized by the centralized BBU pool and the distributed RRHs.
As a common used means of supporting a plurality of base stations independent from each other within a multi-core hardware unit, the virtualization technology allocates sufficient computer resources to individual virtual base stations in accordance with requirements of respective air interface standards to ensure mutual independence of individual virtual base stations at the operating system level.
The virtual base station live migration is a new technology that emerged from C-RAN, which can reduce the power consumption of the C-RAN BBU pool. It has been observed that the workload in a base station is very low during the latter half of the night. Although the base station handles very low workload, it consumes almost the same level of energy as it does in busy hours. Thus, if multiple virtual base stations can be migrated onto a single BBU (Baseband Unit), the idle BBUs may be shutdown, resulting in reduced power consumption of the BBU pool.
In order to achieve a lossless virtual base station dynamic migration, necessary control-plane and user-plane data needs to be forwarded from the source base station to the target base station. Some of the necessary information can be treated in a similar way to a cell handover. For example, the Packet Data Convergence Protocol Service Data Unit (PDCP SDU) buffered data and the PDCP Sequence Number (PDCP SN) in Radio Link Controller-Acknowledged Mode: Data Radio Bearer (RLC-AM: DRB) should be restored at the target base station; user device contexts should be transmitted from the source base station to the target base station, and the user device contexts include user device (UE)/authentication server (AS) Security Information, E-RABs (Radio Access Bearer) information, RRC contexts, etc. In order to reduce the impacts of a base station migration on user devices, some extra user device context information also needs to be transmitted from the source base station to the target base station. This extra user device context information includes: PDCP SDUs buffered data and PDCP SN of RLC-UM DRB (UM: Unacknowledged Mode); the keys (KeNB) of the source base station; RLC contexts and the buffered data of RLC PDU/SDUs; HARQ (Hybrid Automatic Repeat Request) configuration and state, and HARQ buffered data; cell specific parameters such as MIB (Mater Information Block), SIB1-6 (System Information Block), etc. Even so, the detailed information needed to be forwarded to the target base station is not the subject of this invention.
As all the baseband units are in the same BBU pool and are connected with each other by high speed fixed network, the latency between virtual base stations is far lower than the traditional backhaul latency, which facilitates the data exchange between the source base station and the target base station and makes a seamless base station migration possible.
In principle, user devices always suffers from a service interruption when a running virtual base station migrates from one BBU to another, which is caused by the suspension of the source base station in the source BBU, the transfer of the necessary parameters and data from the source base station to the target base station, and the user device context restoration and the activation of the target base station in the target BBU. During the service interruption, neither the source base station nor the target base station serves the user devices. If a user device doesn't receive any Physical Downlink Control Channel (PDCCH) signaling during a period of time, the user device does not know what happens and it may trigger a Radio Resource Control (RRC) re-establishment which may lead to broken services. Thus, in the present invention, it is presented a dynamic mechanism to enable user devices not to be influenced during the service interruption, and ensure the user devices to be served as soon as possible after the migration is completed and guarantee the user device services are not broken.